In various plasma application equipment, glow discharge application equipment and the like, occurrence of arc discharge during discharging often causes problems in the operation of the equipment. For this reason, a discharging power supply is often required that can definitely and quickly break arc discharge. In the following, as a specific example, a sputtering apparatus for use in forming thin film will be described.
FIG. 11 is a schematic diagram showing the structure of a relevant part of a DC sputtering apparatus. This sputtering apparatus comprises a vacuum chamber 101 and a sputtering DC power supply 110. The anode of the power supply 110 is connected via a connecting cable 120A to the chamber 101, and placed at ground potential. On the other hand, the cathode of the power supply 110 is connected via a connecting cable 120B to a sputtering target 104 provided inside the chamber 101. A substrate 100 on which thin film is to be deposited is placed inside the chamber 101.
In forming film, first, the inside of the chamber 101 is evacuated by a vacuum evacuation pump 106, and discharge gas such as argon (Ar) is introduced from a gas supply source 107 to maintain the inside of the chamber at a predetermined discharge pressure. Next, the power supply 110 applies an electric field between the target 104 and the chamber 101 to generate glow discharge 108. Then positive ions in plasma generated in the discharge space impinge on the surface of the target 104 to sputter atoms of the target 104. Such a sputter phenomenon can be used to form thin film made of the material of the target 104 on the substrate 100.
Such a sputtering apparatus is widely used in the process of forming thin film for various products including semiconductor devices, CDs (Compact Discs), DVDs (Digital Versatile Discs), and liquid crystal display devices.
However, arc discharge 150 may occur during such a sputter operation. Such arc discharge 150 occurs more often in the vicinity of the target 104. When such arc discharge 150 occurs, a large current locally flows, which damages the target 104 or the substrate 100.
For example, when arc discharge 150 occurs on the side of the target 104, a large current concentrates on a small region of the target 104, which causes a large amount of deposition material to emit instantaneously from the region. This phenomenon is referred to as “splash”, which splashes particles of deposition material on the surface of the substrate 100, thus causing damage.
It is therefore required to provide arc suppressing means in the power supply 110 in order to prevent damage due to such arc discharge.
Methods of suppressing arc discharge in DC sputtering may include the following:
(1) Arc discharge is suppressed by periodically turning off the power supply output to provide a resting period.
(2) Arc discharge is quenched by inverting the current using the switching action of arc discharge and the oscillation of LC provided in the output circuit of the power supply.
(3) Arc discharge is detected, and the arc current is turned off by a switch element. In this case, the switch element may be inserted in series or parallel with the load to turn off the arc current.
(4) Arc discharge is detected, and then it is quenched by activating a switch element to apply reverse voltage.
(5) Arc discharge is suppressed by periodically activating a switch element to apply reverse voltage.
Among them, the inventors disclosed the methods that use a switch element in Japanese Patent Nos. 2835322 and 2835323.
On the other hand, the above method (1) has a problem of decreasing throughput because sputtering is interrupted.
In contrast, the above method (2) has the potential to simplify the apparatus configuration in that arc discharge can be suppressed without using switch elements and the like. More specifically, if an LC resonant circuit is provided at the output end of the DC power supply, and when arc discharge occurs, its switching action can be used to generate vibrating current. When this vibrating current has crossed 0 A (zero ampere) to change its polarity, the current can be turned off by the rectifying action of arc discharge (current does not flow under the reverse voltage because thermoelectrons are emitted only from the hot spot), thereby quenching arc.
However, after independent investigation, the inventors have found that such a discharging power supply having arc quenching mechanism with vibrating current has room for improvement with respect to its operation.
More specifically, the operation control for such a DC power supply using an LC resonant circuit may be performed so that a constant power control operation, a rated current limit operation, and a rated voltage limit operation are combined to prefer one of the signals that minimizes the output.
In other words, in such operation control, for a low sputter voltage, the flowing current is controlled to have the smaller of a value corresponding to the constant power control operation and a value limited by the rated current limit operation.
In contrast, in order to quench arc in the vibration mechanism, the amplitude of vibrating current that is generated at the transition from sputter voltage to arc voltage and that depends on LC circuit constants must exceed the sputter current to generate current vibration that crosses 0 A.
Two methods for generating such a large current vibration can be mentioned.
The first of the methods is to use LC constants for which a large current amplitude can be obtained corresponding to the rated current limit operation even for a low sputter voltage (e.g., 200 V).
The second is to set LC constants so that current amplitude corresponding to the rated current limit can be obtained at a lower value in the range of normal sputter voltage (e.g., 400 V), and below this sputter voltage, to variably control the power rating so that the upper limit of current is decreased corresponding to the voltage.
However, when the first method is used, an excessively large current vibration occurs during operation at normal sputter voltage (e.g., 600 V). As a result, large stress may be imposed on electric parts used in the power supply and/or noise may occur.
For this reason, the LC constants must be determined to correspond to a lower sputter voltage, and thus the second method described above must be used. For example, the discharge voltage may significantly decrease immediately after the target 104 is attached or after the chamber 101 is opened to the atmosphere because the surface of the target 104 is covered with oxides and the like. For this reason, the sputter power must be set to be sufficiently low. Thus, sputtering is performed initially in a sufficiently low power setting, which is referred to as “target cleaning mode”. Then, as oxides are removed from the surface of the target 104 and the discharge voltage increases, the power setting is raised to transfer to normal sputter voltage.
However, this method has a problem that it involves cumbersome operations because the power setting is adjusted according to the sputter voltage.
The invention has been made on the basis of recognition of these problems. An object of the invention is to provide a discharging power supply and sputtering power supply being capable of quickly and definitely quenching arc discharge, having simple structure and being easy to operate, and a sputtering apparatus using the same.